Abstract

Nucleation and growth of new particles in the atmosphere is thought to account for up to half of all cloud
condensation nuclei. However the vapours and formation rates that underly this process are poorly understood, due both to the ultra low concentrations of participating vapours in the presence of high backgrounds and to the many sources of uncontrolled variability in the atmosphere. In consequence, laboratory measurements made under clean and precisely controlled conditions play an important role in identifying the vapours responsible and quantifying their associated nucleation and growth rates. The CLOUD experiment at CERN is studying the nucleation and growth of aerosol particles, and their interaction with clouds, in a 3 m stainless steel aerosol/cloud chamber. The experiment is optimised to study the influence of ions, for which the CERN Proton Synchrotron (PS) provides an adjustable source of 'cosmic rays'. Extraordinary care has been paid in the design and construction of CLOUD and its associated systems-gas, thermal, UV and electric field-to suppress contaminants at the technological limit. The unprecedented low contamination achieved in the CLOUD chamber has revealed that atmospheric nucleation and growth is sensitive to certain atmospheric vapours at mixing ratios of only a few parts-per-trillion by volume (pptv). Here we provide an overview of the design of CLOUD and its experimental programme over four years of operation at CERN.